Change to Return Air Ducting OK?

My house has a mostly finished basement. We have an additional area that we're looking to finish out as well. Ceiling height to the floor joists above is a touch over 7 feet, which is great, but in the area we want to refinish, the furnace air return duct drops that to about 6' 4". That's still not bad, but it's not 7 feet! The duct goes from a floor grate near the bottom of the stairs on the first floor above, across the ceiling of the area I want to refinish, about 4 feet or so, and then through a brick wall to the furnace. Just before it enters the brick wall, the duct opens up from 8 x 20 or so, to about 16 by 20.

Here's my idea... I'm thinking I could possibly just remove the duct altogether. If I leave the floor grate above in place, open to the basement below, and put a grille across the opening as the duct goes through the brick wall, everything will look great and I'll have my full 7+ feet of ceiling height, but will the heating system still work right? The furnace would draw on the basement space rather than on the area at the base of the stairs on the floor above. It seems to me the basement space would naturally then just draw on the upstairs through that now open floor register, and to some extent through the kitchen and down the stairs. All of that seems totally fine to me. The basement is bone dry, with a solid slab throughout, and totally sealed from the outdoors. We've just installed a radon mitigation system that is working well, so I'm not concerned about that. There are three supply registers in the basement that today return up the stairs and around through the kitchen to the floor grate. It seems like the new setup would probably make those supplies work a little better with a teeny bit less load on the fan as result.

So what am I missing? Would this work? Any problems it might cause?

Thanks in advance for any feedback,

J

Welcome to the site. An open hole between floors is a free run for fires and not a good idea.
You might look at dividing it in half between upstairs and down. For the upstairs one you could remove the duct and put a piece of sheet metal against the joists and add a few 2x4s on edge and drywall that. You would still have 80".